1. Field of the Invention
The present invention relates to a cubic boron nitride radius end mill used for machining of medium or high hardness materials, such as quench-hardened steel, and finish-machining of die surfaces and the like. The term “cubic boron nitride radius end mill” refers to an end mill having a cutting edge composed of a sintered cubic boron nitride compact.
2. Description of the Related Art
An example of a cubic boron nitride radius end mill according to the related art is described in Japanese Unexamined Patent Application Publication No. 2005-118960 (Patent Ref. 1). In a radius end mill disclosed in Patent Ref. 1, a cutting edge is an axially symmetrical, helical tooth in bottom view, and a chamfer (corresponding to a “negative rake face” in the present invention) with a negative angle is disposed on a face along the cutting edge.
The chamfer strengthens the edge. In a tool in which a sintered cubic boron nitride compact is used as a material for a cutting edge, the effectiveness of the chamfer is significant. However, even if the edge is strengthened by the chamfer, the depth of cut cannot be increased during cutting, and therefore, the effect of a helical tooth is not very large. In the case of machining of a high hardness material with an end mill composed of a sintered hard alloy, the surface of which is provided with a hard coating, the life of the end mill is short even at a small depth of cut, and the machined surface tends to be coarse due to the damage to the cutting edge. In order to cope with such problems, cubic boron nitride end mills have been developed. However, with respect to a cubic boron nitride radius end mill in which a quarter-circular radius cutting edge (corner cutting edge) is continuously connected to the outer end of an end cutting edge, and a peripheral cutting edge is continuously connected to the outer end of the radius cutting edge, even if the edge is strengthened by a chamfer, the desired end mill life may not be obtained in some cases.